The present invention relates generally to optical wavelength filters, and in particular to passive and active filters using claddings with a depressed refractive index.
Optical waveguides are designed to guide light of various modes and polarization states contained within a range of wavelengths in a controlled fashion. Single-mode optical fiber is the most common waveguide for long-distance delivery of light. Other waveguides, such as diffused waveguides, ion-exchanged waveguides, strip-loaded waveguides, planar waveguides, and polymer waveguides are commonly used for guiding light over short distances and especially for combining or separating light of different wavelengths, optical frequency mixing in nonlinear optical materials, modulating light and integrating many functions and operations into a small space.
In essence, a waveguide is a high refractive index material, usually referred to as core in optical fiber, immersed in a lower index material or structure, usually referred to as cladding, such that light injected into the high index material within an acceptance cone is generally confined to propagate through it. That is because at the interface between the high and low index materials the light undergoes total internal reflection (TIR) back into the high index material.
The advent of active fiber elements such as Er-doped amplifiers and fiber lasers has resulted in further developments in fiber claddings. For example, fibers with profiles designed for xe2x80x9ccladding pumpingxe2x80x9d have been developed to stimulate lasing in the active core of single-mode fibers. Such fibers typically have a core, a multi-mode cladding layer and an outermost cladding layer. The pumping light is in-coupled into the multi-mode cladding layer and it crisscrosses the core as it propagates thus stimulating lasing. Unfortunately, once the fiber core is stimulated to emit at the desired emission wavelength, the core can also produce an unwanted signal at the so-called xe2x80x9cRaman wavelengthxe2x80x9d due to stimulated Raman scattering. The Raman effect shifts the emission wavelength to the first Stokes wavelength of the fiber core and becomes dominant at high power levels. Hence, the output power of cladding pumped fiber lasers at the desired output wavelength is limited.
Similar problems occur when the active material in the medium has other emission wavelengths that are stimulated along with the desired emission wavelength. This problem is commonly referred to as amplified spontaneous emission (ASE) and may prevent the laser from operating at the desired wavelength by xe2x80x9cdrowning outxe2x80x9d the desired frequency with much higher ASE intensity output at the undesired wavelengths. An example of this situation is encountered when working with neodymium (Nd) fiber lasers having a Nd-doped core surrounded by a double-cladding and using the 4F3/2 to 4I9/2 transition of neodymium atoms to generate light of about 900 nm wavelength. Unfortunately, the 900 nm transition is three-level type and the neodymium atoms have a higher probability to emit light at roughly 1050-1070 nm where the transition is four-level type. The undesired light at 1050 nm easily dominates over the feeble 900 nm emission rendering the fiber laser or amplifier useless for most practical purposes.
Consequently, certain fiber devices such as fiber lasers and especially amplifiers, need to have a high loss at undesired long wavelengths. In the specific example of a Nd:glass laser or amplifier intended to emit or amplify light at 900 nm, it is necessary to obtain very high losses at 1050 nm. That is because the light at 900 nm is very feeble (it is 20 nm away from the peak of the emission at 880 nm), and the emission cross-section at 1050 nm is approximately 10 times the emission cross-section at 900 nm. Also, the light at 900 nm should be amplified, e.g., by 40 dB. Unfortunately, as the light at 900 nm is being amplified by 40 dB the light at 1050 nm is also amplified by 400 dB, thus completely dominating over the light at 900 nm. Clearly, light at 1050 nm needs to be attenuated by 400 dB or more to avoid this problem.
In addition, once about a kilowatt of power is being produced by the fiber laser or amplifier, even if it is just for a nanosecond or less, many nonlinear processes in addition to Raman scattering start interfering with generation of the useful light at 900 nm. These include phenomena such as Brillouin scattering, self-phase modulation as well as a host of other nonlinear optical mixing and conjugation processes via the third order nonlinear susceptibility "khgr"(3) of the fiber material. These effects limit the length of fiber which can be used in such fiber lasers or amplifiers. In fact, due to these effects the fiber length should be on the order of 1 m or even less. This means that light at 1050 has to be attenuated at an attenuation rate of about 400 dB/m.
In view of the above, it is clear that it would be desirable to selectively filter out unwanted wavelengths from a fiber laser or amplifier at a very high attenuation rate, e.g., 400 dB/m or more. This would be important for passive and active waveguides such as passive fibers, fiber lasers and fiber amplifiers.
In accordance with the prior art, waveguides are usually designed to prevent injected light from coupling out via mechanisms such as evanescent wave out-coupling (tunneling), scattering, bending losses and leaky-mode losses. A general study of these mechanisms can be found in the literature such as L. G. Cohen et al., xe2x80x9cRadiating Leaky-Mode Losses in Single-Mode Lightguides with Depressed-Index Claddingsxe2x80x9d, IEEE Journal of Quantum Electronics, Vol. QE-18, No. 10, October 1982, pp. 1467-72. In this reference the authors describe the propagation of light in more complex lightguides with claddings having a variation in the refractive index also referred to as double-clad fibers. They teach that varying the cladding profile can improve various quality parameters of the guided modes while simultaneously maintaining low losses. Moreover, they observe that depressed-index claddings produce high losses to the fundamental mode at long wavelengths. Further, they determine that W-profile fibers with high index core, low index inner cladding and intermediate index outer cladding have a certain cutoff wavelength above which fundamental mode losses from the core escalate. These losses do not produce very high attenuation rates and, in fact, the authors study the guiding behavior of the fiber near this cutoff wavelength to suggest ways of reducing losses.
U.S. Pat. Nos. 5,892,615 and 6,118,575 teach the use of W-profile fibers similar to those described by L. G. Cohen, or QC fibers to suppress unwanted frequencies and thus achieve higher output power in a cladding pumped laser. Such fibers naturally leak light at long wavelengths, as discussed above, and are more sensitive to bending than other fibers. In fact, when bent the curvature spoils the W or QC fiber""s ability to guide light by total internal reflection. The longer the wavelength, the deeper its evanescent field penetrates out of the core of the fiber, and the more likely the light at that wavelength will be lost from the core of the bent fiber. Hence, bending the fiber cuts off the unpreferred lower frequencies (longer wavelengths), such as the Raman scattered wavelengths, at rates of hundreds of dB per meter.
Unfortunately, the bending of profiled fibers is not a very controllable and reproducible manner of achieving well-defined cutoff losses. To achieve a particular curvature the fiber has to be bent, e.g., by winding it around a spool at just the right radius. Different fibers manufactured at different times exhibit variation in their refractive index profiles as well as core and cladding thicknesses. Therefore, the right radius of curvature for the fibers will differ from fiber to fiber. Hence, this approach to obtaining high attenuation rates is not practical in manufacturing.
The prior art also teaches waveguides with complex cladding structures rather than stress-induced birefringence for polarization control. For example, U.S. Pat. No. 5,056,888 to Messerly et al. teaches a single-mode, single-polarization fiber with a generally elliptical W-profile. This radial profile distribution enables one to separate the orthogonal polarizations of light at a given wavelength. The separation occurs because light of one polarization will leak out of the core while light of the orthogonal polarization will not. Additional teaching on this subject can be found in Stolen et al., xe2x80x9cShort W-Tunneling Fibre Polarizersxe2x80x9d, Electronics Letters, Vol. 24, 1988, pp. 524-525; U.S. Pat. No. 4,515,436 to Howard et al. and numerous other references. Unfortunately, such fiber waveguides can not achieve the necessary attenuation rates.
In view of the above, it would be an advance in the art to provide mass-producible wavelength filters having very high attenuation rates, e.g., 400 dB/m or more for undesired wavelengths longer that wavelengths of interest. Such wavelength filters should be adaptable to active and passive optical waveguides such as passive fibers, fiber lasers and fiber amplifiers. It would be an especially welcome advance to provide for precise tailoring of the filter properties without the need to bend or otherwise manipulate and adjust the waveguide or fiber after its production.
In view of the above, it is a primary object of the present invention to provide a method for engineering optical wavelength filters for high losses in a manner that is reliable, reproducible and simple. The method should provide wavelength filters exhibiting losses of 400 dB/m and more at longer wavelengths, i.e., short pass filters, without the need for bending or otherwise manipulating or adjusting waveguides or fibers.
It is a further object of the invention to provide a method for wavelength filtering which can be employed in passive and active waveguiding devices, such as passive fibers, fiber lasers and fiber amplifiers.
It is yet another object of the invention to provide a method and filters for wavelength filtering which are compatible with nonlinear optical devices.
These and numerous other advantages of the present invention will become apparent upon reading the detailed description.
The present invention provides an apparatus and a method for separating light of a first wavelength xcex1 from light of a second wavelength xcex2, where xcex1 less than xcex2. The apparatus has a core exhibiting a certain core cross-section and having a refractive index no. The core is surrounded by a depressed cladding or cladding with a depressed index of refraction n1. The depressed cladding has a depressed cladding cross-section and is itself surrounded by a secondary cladding which has a refractive index n2 and a secondary cladding cross-section. The apparatus is designed in such a manner that a fundamental mode of light propagating within the core at the second wavelength xcex2 is lost or cut off. This is accomplished by selecting the core cross-section, the depressed cladding cross-section, the secondary cladding cross-section and the refractive indices no, n1, n2 to produce a fundamental mode cutoff wavelength xcexc such that xcex1 less than xcexc less than xcex2. In practical applications, the refractive indices no, n1, n2 are values averaged over the corresponding cross-sections.
The sections of the core and claddings can be of any shape. For instance, they can all exhibit the same cylindrical symmetry. In some embodiments, however, cylindrical symmetry may not be appropriate and other symmetries can be selected. For example, in some embodiments the core can have an elliptical cross-section.
In accordance with the invention, the selection of the cross-sections and refractive indices determines the fundamental mode cutoff wavelength xcexc as well as the losses incurred by wavelengths above the cutoff wavelength xcexc. Specifically, these choices determine the losses incurred by light at second wavelength xcex2. In some embodiments, the selection is performed such as to produce a loss of at least 100 dB/m at the second wavelength xcex2. In some embodiments requiring very high losses to the second wavelength, the selection of the cross-sections and indices can be made such that the losses at the second wavelength xcex2 are at least 400 dB/m.
The selection rules for the sections and indices are derived from the Maxwell equations. More specifically, however, it may be convenient to select the refractive indices such that:
{square root over ((n22xe2x88x92n12)/(no2xe2x88x92n22))}xe2x89xa70.224. 
In another embodiment, it may also be convenient to select the refractive indices such that:
{square root over ((n22xe2x88x92n12)/(no2xe2x88x92n22))}xe2x89xa73. 
When the core has a section radius r0, and the depressed cladding section has an outer radius r1, it may also be convenient to select the refractive indices such that:             r      1              r      o        ≥      1    +                                        (                                          n                o                2                            -                              n                2                2                                      )                    /                      (                                          n                2                2                            -                              n                1                2                                      )                              .      
The apparatus can have a core with an optically active material at the first wavelength xcex1. For example, the optically active material is a nonlinear optical material or a gain medium having a first gain at xcex1. In some cases the apparatus can be further equipped with an optical feedback to the gain medium thereby creating a laser emitting at xcex1. Such laser can be further equipped with a secondary cladding surrounding the outer cladding and a pump source optically coupled to the secondary cladding for supplying pump light to the core. Without the feedback the apparatus can act as an optical amplifier for amplifying light at xcex1. In another embodiment, the apparatus acts as a broadband amplified spontaneous emission source of light near xcex1.
Whether the apparatus is set up as a laser or amplifier, the core frequently has a second gain at second wavelength xcex2. In this case the cross-sections and indices of refraction are selected to produce loss at the second wavelength xcex2 at least equal to the second gain. Preferably, the loss at xcex2 is at least equal to 100 dB/m or even 400 dB/m or more.
The gain medium can have at least one rare earth element selected from the group consisting of Neodymium, Ytterbium, Erbium, Thulmium and Holmium. In a preferred embodiment, when Erbium is used as the gain medium and the apparatus functions as an optical amplifier at xcex1, the cut-off wavelength xcexc can be set in the middle, or toward the long wavelength end, of the S-band. This configuration is particularly useful for telecommunications applications.
In some embodiments the core will have an optically active material producing light at xcex2 or having second gain at xcex2. In some cases the optically active material may also exhibit a third gain at a third wavelength xcex3, where xcexc less than xcex3. In such devices both xcex2 and xcex3 are filtered out of the core. This situation is encountered, for example, when the second gain is due to stimulated emission of the optically active material or gain medium at xcex2 and the third gain is due to stimulated Raman scattering of light at xcex1.
In accordance with yet another embodiment of the invention, the apparatus can be rendered polarization-selective. For example, the light at xcex1 may have a first polarization and a second polarization orthogonal to the first polarization. One or more of the cross sections and refractive indices are further adjusted in this embodiment such that xcex1 less than xcexc for the first polarization and xcex1 greater than xcexc for the second polarization. In other words, the first polarization at xcex1 is cut off from the core along with wavelength xcex2. In this same or another embodiment the core, depressed cladding and outer cladding can form a polarization maintaining fiber.
The invention also provides a method for separating light of first wavelength xcex1 from light of second wavelength xcex2, where xcex1 less than xcex2. This is done by engineering the cross-sections of the core, depressed cladding and secondary cladding as well as their refractive indices to ensure that the fundamental cutoff wavelength xcexc to the fundamental mode of light at these wavelengths is such that xcex1 less than xcexc less than xcex2. The losses to xcex2 are adjusted to 100 dB/m or even 400 dB/m or more by appropriate choice of the cross-sections and refractive indices.
Optionally, the secondary cladding can be surrounded by an outer cladding, and can be optically coupled to a pump source for guiding pump light that excites the gain medium in the core. This method is particularly advantageous when the method is applied to a fiber amplifier or a fiber laser with optical feedback to the gain medium.
In situations where a particular polarization of light at xcex1 has to be cut off along with xcex2, the method further provides for adjusting the cross-sections and/or indices of refraction. Furthermore, in those situations stress can be applied to render the core and/or depressed cladding polarization selective.
A detailed description of the invention and the preferred and alternative embodiments is presented below in reference to the attached drawing figures.